Ultrasonic flowmeters of the type described above, which determine the volume flow rate on the basis of the so-called travel time method, are used in many cases in process- and automation-technology. These flowmeters permit the volume flow rate in a containment, e.g. in a pipeline, to be measured contactlessly.
There is a differentiation between ultrasonic flow rate sensors, which are inserted into the pipeline, and clamp-on flowmeters, in which the ultrasonic transducers are clamped externally onto the conduit by means of a clamping mechanism. Clamp-on flowmeters are described, for example, in EP 0 686 255 B1, U.S. Pat. Nos. 4,484,478, or 4,598,593.
In the case of both kinds of ultrasonic flowmeters, the ultrasonic measuring signals are radiated, at a predetermined angle, into the containment in which the medium is located. In the case of inserted ultrasonic flow rate measurement pickups, the particular position of the ultrasonic transducer on the measuring tube is dependent on the inner diameter of the measuring tube, and on the velocity of sound in the medium. Since the inner diameter of the measuring tube is known from the fabrication, depending on application, at most the velocity of sound in the medium remains an only approximately known parameter.
In the case of clamp-on flowmeters, the wall thickness of the pipeline and the velocity of sound in the material are added as further application parameters. Relatively large errors can be associated with these parameters.
Depending on the application, in the case of clamp-on flowmeters, a still further source of error occurs. An ultrasonic transducer, which is used in the case of a clamp-on flowmeter, includes at least one piezoelectric element producing the ultrasonic measuring signal, and a coupling wedge. This coupling wedge is normally fabricated out of plastic, and, on one hand, serves for impedance matching and, on the other hand, for the protection of the piezoelectric element.
The ultrasonic measuring signals produced in a piezoelectric element are conducted through the coupling wedge, or a lead-in member, and the pipe wall, into the liquid medium. Since the velocity of sound in a liquid and in plastic are different from one another, the ultrasonic waves are refracted at the transition from one medium into the other. The angle of refraction itself is defined according to Snell's Law, that is, the angle of refraction depends on the ratio of the propagation velocities of the two mediums.
With coupling wedges, or lead-in members, made of plastic, a good impedance matching can be achieved; however, the velocity of sound in plastic exhibits a relatively high dependency on temperature. Typically, the velocity of sound in plastic varies from circa 2500 m/s at 25° C. to circa 2200 m/s at 130° C. In addition to the temperature-induced change in travel time of the ultrasonic measuring signals in the plastic of the coupling wedge, the direction of propagation of the ultrasonic measuring signals in the flowing medium also changes. Thus, both changes have an unfavorable effect on the accuracy of measurement of an ultrasonic transducer which functions according to the travel time method.
In the case of known flowmeters, the angular positioning of the ultrasonic transducers is predetermined. For the purpose of initial mounting, or in the case of later changes in application, it is necessary, on the basis of the above considerations, to adjust the mutual separation of the two ultrasonic transducers in a defined manner. For this, normally one of the two ultrasonic transducers is moved relative to the other until the position is established in which the intensity of the measuring signals received by the ultrasonic transducers is maximum. After the optimum separation of the two ultrasonic transducers is established by this trial-and-error method, the two ultrasonic transducers are then locked securely to the pipe wall in such established positioning. This method, of course, is relatively time consuming.
In addition to this, some of the application parameters, which, in the case of a clamp-on flowmeter, are necessary for the exact determination of the volume flow rate, are precisely known only in the rarest of cases; or, however, the establishment of these parameters is rather complex. While the establishment of the exterior diameter of the pipeline hardly causes problems, the exact determination of the wall thickness of the pipeline can be absolutely problematic. Furthermore, in many cases, neither the velocity of sound in the material of the pipeline, nor the velocity of sound in the medium is exactly known.